| MWP |
Hi all,
My new DAC+AMP is based around this simple circuit:
The DAC outputs the audio on a 1.6V bias volatge.
The design was meant to use the pot to trim out this voltage on the first opamp.
But no matter what its set to, i always get a -0.7V dc-offset on the OPA627 ouput.
This then leads to a +12V dc-offset on the LM3886's output.
Can anyone see what is wrong here?
I really dont want to put DC blocking caps after the OPA627 to fix this.
Thanks for any help. |
|
|
| Nuuk |
| Shouldn't that 10K from non-inverting to ground be 180K? :att'n: |
|
|
| analog_sa |
Circuit looks fine to me. Are you measuring the voltage at the positive input?
Your speakers will enjoy life close to the edge :) |
|
|
| MWP |
| quote: | Originally posted by Nuuk
Shouldn't that 10K from non-inverting to ground be 180K? :att'n: |
I dont think so.
It wouldnt really make a difference anyway.
| quote: | Originally posted by analog_sa
Circuit looks fine to me. Are you measuring the voltage at the positive input?
Your speakers will enjoy life close to the edge :) |
The positive input? |
|
|
| analog_sa |
| Just to make sure that the pot/bias source are correctly connected. The negative input then will have little choice but follow. |
|
|
| Yoghourt |
| What are supply voltages for your OPA627 and your LM3886? |
|
|
| MWP |
| quote: | Originally posted by analog_sa
Just to make sure that the pot/bias source are correctly connected. The negative input then will have little choice but follow. |
So my design is correct then?
If its built correctly (which im 99% sure it is as 4 of these circuits do the same thing) what could it be?
| quote: | Originally posted by Yoghourt
What are supply voltages for your OPA627 and your LM3886? |
OPA627 is 12-0-12.
LM3886 is 35-0-35. |
|
|
| MWP |
Strange... it needed 2.4V on the OPA627's + pin.
All fixed, can now get ~5mV on the LM3886 output.
Thanks. |
|
|
| drdagor |
The op amp bibles say to use a servo to control offset. There are some good notes on designing servos in these notes.
Essentially, provide negative feedback through a low pass filter -- very low pass (like 8 Hz for f0) and it will lock the output of the first stage to ground.
Be sure to use a low noise opamp in the feedback circuit, because you are applying the output to one of the inputs of your gain stage. |
|
|
| drdagor |
The op amp bibles say to use a servo to control offset. There are some good notes on designing servos in these notes.
Essentially, provide negative feedback through a low pass filter -- very low pass (like 8 Hz for f0) and it will lock the output of the first stage to ground.
Be sure to use a low noise opamp in the feedback circuit, because you are applying the output to one of the inputs of your gain stage. |
|
|
|
